428 
PACIFIC SCIENCE, Vol. XVII, October 1963 
a predecline average of 30 individuals to 800 / 
O. 1 m 2 during the decline. This dropped back 
to an average of 350 living individuals per 0.1 
m 2 by February, I960. Curiously, Balanus tri- 
gonus populations remained unchanged through- 
out the period, as did those of Pachycheles rudis, 
P. holosericus, and Crangon dentipes. Four spe- 
cies of crustaceans disappeared during the My- 
tilus decline: Paraxanthias taylori, Cirolana 
harfordi , Lophopanopeus leucomanus, and L. 
diegensis, the latter of which had been repre- 
sented only by young. These losses were more 
than offset by the appearance of six species, 
among which are Pelia clausa and Pugettia dalli. 
Among the smaller echinoderms, Ophiactis 
simplex reacted quickly by nearly doubling its 
population at the 2-m level, going from 180 
individuals to 340/0.1 m 2 by October, 1959. 
This dropped back to an average of 154/0.1 m 2 
by February, I960. The plankton-feeding holo- 
thuroid Cucumaria lubrica appeared on the 2-m 
level for the first time. 
While working with the above data I was 
struck by the fact that throughout the entire 
period from November, 1958, to February, I960, 
over 72% of the numerable individuals at the 
2-m level were found in only four species (Table 
3). We see, however, that the relative percent- 
ages that each contributed to the total shifted 
considerably during the decline. 
Now let us turn to an analysis of shifts among 
the larger species existing on Reef 200. Not 
only did populations of some of these decline 
significantly, but there were also shifts of popu- 
lation centers on the vertical axis. One might 
easily have predicted some, but not all, of these 
changes. Prior to the decline, 70% of the Pisas- 
ter giganteus population was found on the upper 
half of the reef in close association with Mytilus 
(Table 4). Subsequently this percentage dropped 
to 46. At the same time the Pisaster population 
on the entire reef dropped about 45%, presum- 
ably as a result of emigration to new sites. Most 
of the Pisaster emigrants must have come from 
the upper reef. Quite unexpectedly the sea urchin 
Strongylocentrotus purpuratus appears to have 
been markedly affected by the Mytilus loss. Its 
population dropped 34% and data in Table 4 
point also to a downward movement on the reef. 
Strongylocentrotus franciscanus, on the other 
hand, suffered only a small loss of total popula- 
tion, and the center of population remained on 
the lower reef as before. The quadrat densities 
of the sea cucumber Parastichopus parvimensis 
dropped 30% during the Mytilus decline, but 
the center of population remained on the lower 
half of the reef. 
DISCUSSION 
Four kinds of population changes occurred 
during the Mytilus decline: (1) some species 
previously present with the mussel increased in 
numbers and held their gains; (2) some species 
TABLE 3 
Percentage of Total Individuals of Numerable Species on 
Each Quadrat Provided by Only Four Species 
SPECIES* 
PERCENTAGES OF 
TOTAL POPULATION BY DATEf 
11/21/58 
1/16/59 
9/4/59 
10/4/59 
2/14/60 
Mytilus edulis 
61 
62 
10 
5 
0.2 
Balanus tintinnahulum 
2 
2 
40 
46 
42 
Nereis grubei 
0.2 
0.7 
4 
5 
10 
Ophiactis simplex 
11 
10 
20 
22 
20 
PERCENTAGE OF POPULATION 
74.2 
74.7 
74 
78 
72.2 
NO. OF 
OTHER NUMERABLE SPECIES 
53 
50 
50 
26 
68 
* Note that these four species accounted for an average of 75% of the total animal population, but their relative contribu- 
tions to the total changed markedly during the decline. 
t Data obtained prior to (11/21/58) and during the Mytilus decline on the dates heading columns. All quadrats situated 
at depths of 2 m and within 3 m of the first. 
